1. Field of the Invention
This invention relates generally to a tape tension servo-system for video tape recording and/or reproducing apparatus.
2. Description of the Prior Art
A video tape recording and/or reproducing apparatus (VTR) has been provided with a tape tension servo-system in which an arm mounted for angular movement carries a post engaged by the tape, for example, between the supply reel and the so-called head drum, so that the tape tension can be detected from the angular position of the arm, whereupon a motor coupled with the supply reel is driven in accordance with the detected tape tension so as to maintain the latter at a desired value. In the known VTR, the supply reel motor is electric current driven in accordance with the detected tape tension. In the case of an electric current drive, the motor torque is controlled for achieving the desired tape tension so as to permit a necessary torque to be established in advance. By reason of the foregoing, in the case of an electric current drive, the supply reel motor can be controlled, at least to some extent, even if a strong feedback servo is not obtained.
However, an electric current drive is highly susceptible to external disturbances, such as, vibrations and rolling or changes in orientation of the motor, as is inevitable in the case of a portable VTR.
Although it is known that a voltage driven motor is not susceptible to such external disturbances, a voltage drive serves to control the motor speed, and hence a desired motor torque cannot be established in advance. The existing tape tension detecting devices are neither sufficiently responsive nor sufficiently accurate to permit the use of a voltage driven motor controlled in response to the detected tape tension deviations for driving the supply reel.
It is known also to provide a VTR with a central processing unit (CPU) or micro-processor for determining the drive signals to be supplied to the servo-systems for the supply and take-up reels, the capstan and the head drum of the VTR. In such case, numerous counters are required for detecting various phase differences or time differences to be used by the CPU in determining the various drive signals for the several servo systems on the basis of phase (PG) pulses and frequency (FG) pulses provided by respective generators associated with the supply and take-up reels, the capstan and the head drum, respectively. However, the use of individual counters for determining the various phase and time differences undesirably increases the complexity and cost of the hardware or hard-wired components required for the servo-systems. Further, by employing the CPU for all calculations required in connection with the operation of the various servo-systems, an undesirably heavy load is imposed on the CPU which is thereby unavailable for performing other functions.